The present invention relates to a sintered alloy for use in a valve seat for an intake/exhaust valve of an internal combustion engine.
Valve seats made of sintered alloys have been widely used in internal combustion engines because of their superior wear resistance since the advent of lead-free gasoline. However, the presence of sinter cells or pores which contribute to the superior wear resistance of a valve seat formed of such a sintered alloy presents problems with respect to the strength of the valve seat. These cells or pores can be continuous or closed and hereinafter are referred to as "cells".
When a valve seat is mounted on a cylinder head of an aluminum alloy by techniques such as shrinkage-fit, expansion fit, or by application of pressure, the valve seat is prevented from dropping from the cylinder head as long as the valve seat has an appropriate thickness. When, however, the valve open area of the cylinder head is increased in order to increase the engine output, it is necessary to decrease the thickness of the valve seat. In this case, problems such as dropping or deformation of the valve seat inevitably develop. In engines such as a Diesel engine in which a head made of cast iron is used, the difference in coefficient of thermal expansion between the valve seat and the cast iron cylinder head may sometimes cause the problem of valve seat dropping.
In the case of a valve seat to be mounted at the exhaust side, infiltration with a copper alloy, which serves to increase thermal conductivity and to seal cells, is at times used for the purposes of decreasing accumulation of heat due to exhaust gas and for increasing valve seat strength.
In making such a sintered alloy material, hard grains and cells are dispersed in a base structure of an iron-base alloy. As these hard grains, Fe-Mo and stellite alloy grains are most widely used. An oxide coating is formed in the cells, and wear resistance is increased by the synergistic effect of the hard grains and cells. In general, therefore, the amount of hard grains is about 20% by volume, and the amount of cells is about 15% by volume. Since sintered alloy valve seats are described in, for example, Japanese Patent Publication Nos. 13093/76 and 44947/81. In conventional sintered alloy valve seats, although the wear resistance is good, strength and rigidity are poor since the amount of cells and the amounts of hard grains are large. Thus, it is considered that there is a problem of the valve seat dropping. When infiltration of a copper alloy is applied, for the purpose of increasing strength, the amount of copper used for infiltration is high due to the large amount of cells present. Thus, owing to the difference in coefficient of thermal expansion between the sintered alloy and the infiltrated copper alloy, the rigidity and strength of the valve seat deteriorate when the valve is subjected to a cycle of heating and cooling at high temperature. Furthermore, wear resistance is reduced.